The invention relates to a device for the superposition of rotational speeds as well as to a method therefore.
The invention relates to a device with an auxiliary drive for the rotational speed superposition for a vehicle steering system in which, according to the determined operating state of the vehicle and the steering interventions desired by the driver, the superposition takes place of the control intervention on the steering wheel by the driver and the rotational angle of the auxiliary drive onto the steering movement of the wheels. Through the variable, adjustable or regulatable rotational speed superposition during the steering process, consequently, the control behavior of the vehicle can be optimally adapted to the various driving states. The function of a rotational speed superposition during the steering process is consequently decoupled with respect to a force coupling, rotational moment or torque coupling, such as serves in a servo steering to facilitate the control process, thus a separate independent process which fulfills another function.
Several such devices are already known in prior art. Apart from the application of planetary gearing for the superposition of the rotational angles, or the rotational speed, strain wave gearings, which are also known by the term harmonic drive gearing or pulsator gearing, are also employed.
EP 1 338 493 A1 and JP 2 003 306 155 A introduce a device for the rotational speed superposition for a steering system in which the rotational number superposition takes place with a strain wave gearing. The device therein is implemented integrally and rotationally with the steering shaft driven by the steering wheel and comprises a drive motor whose driving rotational speed is adjustable for the purpose of attaining the desired rotational speed superposition. In the introduced solution the housing is connected with the steering shaft and, consequently, rotates jointly with it. The motor unit disposed in the housing with harmonic drive gearing is consequently also disposed such that it rotates with the steering shaft. A similar disposition has moreover been introduced in DE 19 823 031 A1, corresponding to U.S. Pat. No. 6,164,150, and here the rotational speed superposition device is illuminated with the problematics of locking, in particular in the case of the forward and backward rotation on the steering wheel.
However, the solutions introduced in prior art have several disadvantages. First, the introduction of the electric energy for operating the driving motor must take place into a device which is rotationally supported with respect to the vehicle body. This requires considerable expenditures.
Further, a locking mechanism must be activated which, in the event of an error, for example in the case of a failure of the power supply, connects the input shaft with the output shaft of the device in order to ensure with certainty the control of the vehicle. The solution presented in prior art has the further disadvantage that while actuating the steering wheel the driver must always also set into motion the entire device with the housing for the rotational speed superposition. Through the moved masses, which are greater compared with steering systems without rotational speed superposition, handling becomes more difficult and inert. The responsiveness of the control of the motor vehicle is thereby impaired. A further disadvantage comprises that a large number of structural elements is necessary for the realization of such a rotational speed superposition, which entails higher accuracy requirements made of the parts and higher production costs as well as a decrease of the operating reliability of the configuration.